1. Field of the Invention
The present invention relates to a control apparatus and a control method for a vibration wave driven apparatus such as a vibration-type actuator.
2. Description of the Related Art
Heretofore, as disclosed in Japanese Laid-open Patent Publication No. 10-210775, there have been various proposals for a vibration-type actuator that produces an oval motion of predetermined mass to thereby drive a driven body. To this end, a construction such as for example shown in FIG. 15 is known.
FIG. 15 shows in external perspective view an example of the basic construction of a prior art vibration-type actuator.
As shown in FIG. 15, the vibration-type actuator has a vibrator that includes an elastic body 4 made of a metal material and formed into a shape of a rectangular plate. A piezoelectric element (electro-mechanical energy conversion element) 5 is joined to a rear surface of the elastic body 4, and a plurality of protrusions 6 are provided at predetermined positions on an upper surface of the elastic body 4.
With this construction, AC voltages are applied to the piezoelectric element 5 to simultaneously produce both a second-order bending vibration in a long-side direction of the elastic body 4 and a first-order bending vibration in a short-side direction of the elastic body 4, whereby an oval motion of the protrusions 6 is excited. The oval motion of the protrusions 6 is able to rectilinearly move a driven body 7, if the driven body 7 is made in pressure contact with the protrusions 6. The protrusions 6 therefore function as a driving section of the vibrator.
FIG. 16 schematically shows an example of polarization regions on the piezoelectric element 5 of the vibration-type actuator in FIG. 15. FIGS. 17A and 17B show in perspective view vibration modes of the elastic body 4. FIG. 18 is for explaining an oval motion excited in the protrusions 6 of the elastic body 4.
As shown in FIG. 16, the piezoelectric element 5 is polarization-processed and has two electrodes A1, A2. When in-phase AC voltages V1, V2 are applied to the two electrodes A1, A2, a first-order bending vibration having two nodes each extending in a direction parallel to the long-side direction is excited in the rectangular elastic body 4. This first-order bending vibration corresponds to a first vibration mode shown in FIG. 17A.
When reverse-phase AC voltages V1, V2 are applied to the two electrodes A1, A2, a second-order bending vibration having three nodes each extending parallel to the short-side direction of the rectangular elastic body 4 is excited, which corresponds to a second vibration mode shown in FIG. 17B.
By a combination of the first and second vibration modes, an oval motion is excited in the protrusions 6. The driven body 7 if made in pressure-contact with the protrusions 6 can rectilinearly be driven.
In the first vibration mode shown in FIG. 17A, a vibration of the protrusions 6 is excited such that the amplitude (hereinafter referred to as the Z-axis amplitude) of vibration varies in a direction perpendicular to the surfaces of the protrusions 6 with which the driven body 7 is made in pressure contact. In the second vibration mode shown in FIG. 17B, a vibration of the protrusions 6 is excited such that the amplitude (hereinafter referred to as the X-axis amplitude) of vibration varies in a direction parallel to the direction in which the driven body 7 is driven.
A combination of the first and second two vibration modes is able to excite an oval motion in the protrusions 6, as shown in FIG. 18. A ratio of the Z- and X-axis amplitudes represents an oval ratio of the oval motion.
FIG. 19 shows respective changes in the amplitude of vibration of the protrusions 6 in the first and second vibration modes, which are observed when the phase difference between the two-phase voltages V1, V2 is changed in a range from −180 degrees to 180 degrees.
When the phase difference between the two-phase AC voltages V1, V2 applied to the two electrodes A1, A2 of the polarized piezoelectric element 5 varies from −180degrees to 180 degrees, the amplitude of vibration of the protrusions 6 varies as shown by curves P1, P2 in FIG. 19 in the first and second vibration modes. In FIG. 19, the phase difference is taken along the abscissa, and the amplitude of vibration in the first and second vibration modes is taken along the ordinate.
The oval ratio of the oval motion excited in the protrusions 6 by a combination of the first and second vibration modes can be adjusted by changing the phase difference between the AC voltages V1, V2. In a lower part of FIG. 19, there are shown oval shapes corresponding to phase differences shown below the abscissa therealong.
By changing the phase difference between the AC voltages V1, V2 such that the sign of the phase difference is changed between plus (+) and minus (−), it is possible to change the direction toward which the driven body 7 is rectilinearly driven by the vibration-type actuator. In addition, the driving direction and the driving speed of the actuator can continuously be changed by continuously changing the phase difference, including the plus/minus sign, from an arbitrary value (for example, by continuously changing the phase difference, including the plus/minus sign, from 90 degrees to −90 degrees).
As generally known, the driving speed can be made faster by setting the frequency of the AC voltages applied to the piezoelectric element to a value closer to a resonance frequency of the vibrator, and the driving seed can be made slower by setting the frequency of the AC voltages to a value more far away from the resonance frequency of the vibrator.
A relation between drive frequency and driving speed in the vibration-type actuator having the basic construction shown in FIG. 15 is represented as shown for example in FIG. 20. Specifically, the actuator has a characteristic that the driving speed has a peak value at a resonance frequency of the vibrator, gradually decreases on a frequency side higher than the resonance frequency, and sharply decreases on a frequency side lower than the resonance frequency.
As described above, with the vibration-type actuator having the piezoelectric element 5 polarization-processed as shown in FIG. 16, speed control (frequency control) can be carried out by changing the frequency of the two AC voltages V1, V2 applied to the piezoelectric element 5. Speed control (phase difference control) can also be carried out by changing the phase of the AC voltages V1, V2.
Next, another example of polarization regions in the piezoelectric element 5 of the vibration-type actuator shown in FIG. 15 will be described.
FIG. 21 schematically shows another example of polarization regions of the piezoelectric element 5 of the vibration-type actuator in FIG. 15.
As shown in FIG. 21, the piezoelectric element 5 in this example is polarization-processed and has electrodes A1, A2. The electrodes A1 include piezoelectric regions which are polarization-processed to the “plus (+)” polarity and piezoelectric regions which are polarization-processed to the “minus (−)” polarity.
When the AC voltage V2 is applied to the electrode A2, which is in the piezoelectric region shown in FIG. 21, a first-order bending vibration is excited that corresponds to the first vibration mode shown in FIG. 17A. When the AC voltage V1 is applied to the electrodes A1 in the piezoelectric regions shown in FIG. 21, a second-order bending vibration is excited that corresponds to the second vibration mode shown in FIG. 17B.
With use of the AC voltages V1, V2 of the same frequency but 90 degrees shifted in phase to each other, an oval motion of the protrusions 6 is generated. Thus, it is possible to rectilinearly drive the driven body 7, which is made in pressure contact with the protrusions 6.
The magnitude of the Z-axis amplitude in FIG. 17A can be adjusted by adjusting the amplitude of the AC voltage V2 applied to the electrode A2, and the magnitude of the X-axis amplitude in FIG. 17B can be adjusted by adjusting the amplitude of the AC voltage V1 applied to the electrodes A1. The magnitudes of the X- and Z-axis amplitudes can be adjusted by adjusting a duty ratio of the AC voltages V1, V2 applied to the electrodes A1, A2 by using a digital circuit or a logic circuit.
The moving speed of the driven body 7 can be made faster by increasing the X-axis amplitude (shown in FIG. 18) of the oval motion excited in the protrusions 6, and the moving speed of the driven body 7 can be made slower by decreasing the X-axis amplitude of the oval motion. By variably changing the amplitude of the voltage V1 applied to the electrodes A1 in this manner, the X-axis amplitude of the oval motion can be changed, as shown in FIGS. 22A and 22B, whereby the speed control can be carried out.
With the vibration-type actuator having the piezoelectric element 5 polarization-processed as shown in FIG. 21, the speed control (frequency control or voltage control) can be achieved by changing the frequency or the amplitude of the two AC voltages V1, V2 applied to the piezoelectric element 5, as described above.
By combining the frequency control, the phase difference control, and the voltage control, it is therefore possible to carry out the position control for the vibration-type actuator.
Heretofore, to satisfy both the positioning accuracy and the dynamic range of the position control in driving the vibration-type actuator, a combination of frequency control and phase difference control or a combination of frequency control and voltage control has been proposed. The frequency control is not high in positioning accuracy but excellent in dynamic range. On the other hand, the phase difference control and the voltage control do not have a wide dynamic range but excellent in positioning accuracy. In view of this, it has been proposed to carry out the frequency control for rapid and rough position control and then carry out the phase difference control or the voltage control for accurate positioning.
The present inventors found that the resonance frequency increases with the decrease in amplitude of oval motion in a direction in which the driven body is moved. This is because the elastic body of the vibrator has a nonlinear characteristic with respect to the rigidity. The rigidity of the elastic body changes with the change in amplitude of displacement, resulting in a change in the resonance frequency of the vibrator. Therefore, even when the drive frequency is kept unchanged, the driving speed rapidly decreases (hereinafter referred to as the “rapid deceleration phenomenon”), if the drive frequency decreases to a value lower than the resonance frequency due to a change in phase difference between driving signals or due to a change in the driving signal voltage. Thus, there occurs a dead zone even in the phase difference control or the voltage control, which are high in positioning accuracy.
FIG. 23 shows in graph a relation between drive frequency, driving speed, and phase difference between two-phase voltages applied to the piezoelectric element (i.e., a relation between the phase difference and the resonance frequency of a vibrator). As shown for example in FIG. 23, the more deviated from 90 degrees the phase difference between the voltages V1, V2 applied to the electrodes A1, A2 of the piezoelectric element 5 in FIG. 16, the smaller the amplitude of vibration in the second vibration mode will be and the higher the resonance frequency of the vibrator in the second vibration mode will be. If the drive frequency is set to the resonance frequency at the phase difference of 60 degrees in a state that the phase difference is set at 90 degrees, the drive frequency is at a value higher in frequency than the resonance frequency at the phase difference of 90 degrees, and therefore the rapid deceleration phenomenon does not occur. However, if the phase difference is shifted to a value smaller than 60 degrees, with the drive frequency kept unchanged at the resonance frequency at the phase difference of 60 degrees, the drive frequency becomes lower than the resonance frequency at the phase difference of less than 60 degrees, and the rapid deceleration phenomenon suddenly occurs.
The following is a description of another example. FIG. 24 shows in graph a relation between the duty ratio of AC voltages applied to the piezoelectric element and the resonance frequency of a vibrator. As shown in FIG. 24, the smaller the amplitude (duty ratio) of the voltage V1 applied to the electrode A1 of the piezoelectric element 5 in FIG. 21, the smaller the amplitude of vibration (driving speed) in the second vibration mode and the higher the resonance frequency of the vibrator in the second vibration mode will be. If the drive frequency is set to the resonance frequency at the duty ratio of 30% in a state that the duty ratio is set at 50%, the drive frequency is at a value higher in frequency than the resonance frequency at the duty ratio of 50%, and therefore the rapid deceleration phenomenon does not occur. However, if the duty ratio is shifted to a value smaller than 30%, with the drive frequency kept unchanged at the resonance frequency at the duty ratio of 50%, the resonance frequency at the duty ratio of less than 30% is shifted to the side higher than the fixed drive frequency, and the rapid deceleration phenomenon suddenly occurs.
As described above, the rapid deceleration phenomenon causing a sudden decrease in the driving speed can occur due to a shift of the resonance frequency to a frequency higher than the fixed drive frequency during the execution of the phase difference control or the voltage control, which are high in positioning accuracy. This results in a possibility that a dead zone is caused in the control by the vibration wave driven apparatus.